gitk: Add Brazilian Portuguese (pt-BR) translation

Translating a SCM is tricky due to amount of jargon, so, I tried to
keep the wording consistent with both the German and Italian git
translations and the pt-BR translation of other SCMs.
Signed-off-by: NAlexandre Erwin Ittner <alexandre@ittner.com.br>
Signed-off-by: NPaul Mackerras <paulus@samba.org>